Calorie restriction or caloric restriction, usually abbreviated to CR, is a strategy proven to extend healthy, average, and maximum life span in many short lived species, including mice and rats, and at least healthy and average life span in primates. In research papers it is more usually called dietary restriction, abbreviated to DR, and rodent studies conducted over the past 20 years have reliably demonstrated up to a 40% increase in maximum life span through life-long DR.
These benefits to health and longevity have been shown in animal studies to roughly scale with the degree of calorie restriction imposed, but there is good reason to believe that any gain in primate (and especially human) life span through CR is much more modest than that observed in mice. The calorie restriction response exists in near all species tested to date, and probably evolved very early in the history of life on Earth as a way to increase the chances of surviving seasonal famines or other periodic shortages. Such shortages are the same length whether you are a mouse living a few years or a man living for decades, but for the mouse a season is a much greater fraction of a life span, and thus only the mouse evolved dramatic extension of life in response to famine.
While human calorie restriction doesn't have the same impact on life span, it does provide numerous benefits, such as a greatly lowered risk for most degenerative conditions of aging, and improved measures of health. In recent years, human studies of long-term and short-term calorie restriction have comprehensively demonstrated these benefits. Many researchers believe that the evidence to date shows the practice of CR will in fact extend the healthy human life span, but there simply isn't enough data yet to pin down the effects on life expectancy. It is plausible that they are at least as good as those resulting from exercise. If so, it could mean a difference of 5-10 years of life.
Calorie Restriction Research
The beneficial effects of CR in laboratory animals have been known for more than 80 years, but only in the past decade has an appreciable level of funding and attention been given to this field. Human studies such as CALERIE have been underway for years and many research groups are digging into the operating details of cells and metabolism to firstly explain how the CR response works to extend life, and secondly to try to produce treatments that can mimic this effect. So far a great deal has been learned, but little headway has been made towards calorie restriction mimetic therapies. The genes and processes that control metabolism are notoriously complex, and scientists do not yet have a complete understanding of even this one narrow slice of the bigger picture.
So what is known at present? Loss of visceral fat tissue should be mentioned in the context of CR, as we all know that if you eat fewer calories, you will tend to slim down. A mountain of research indicates that carrying excess body fat is harmful to your long term health in many different ways. Even modest levels of excess weight increases the risk of later suffering common age-related conditions such as diabetes, cancer, and Alzheimer's, with one of the contributing factors being the relationship between fat cells and chronic inflammation. It is no exaggeration to say that if you are overweight, you will have a shorter, less healthy life. This is repeated by the scientific community in study after study. Given all of this, it is plausible that some portion of the health benefits of CR stem from the accompanying loss of fat tissue, although biochemical research indicates that there is clearly more than just that going on under the hood. CR is also creating a variety of positive changes in the controlling mechanisms of metabolism.
For example, it appears that CR provides a boost to the processes of autophagy. Autophagy is the way in which cells remove damaged components in order to recycle the materials into new replacement parts. Several lines of research indicate specific types of damaged cellular components left to cause problems over time contribute to age-related decline and damage inflicted upon the rest of your body's machinery. A greater level of autophagy may help reduce this contribution to the aging process, and thereby extend life.
Practicing Calorie Restriction
How to get started on CR? Fortunately it isn't hard: a wealth of information and many, many starting points exist out there. A restricted diet of this sort aims to reduce the intake of calories to a level 20-40% lower than is typical, while still providing all the necessary nutrients and vitamins. With this in mind, CR is sometimes called "calorie restriction with optimal nutrition" or CRON, and its practitioners have accumulated many years of experience in how best to achieve this end. Good books and a supportive community exist to help newcomers adopt the best practices for CR in humans. Mild CR can be as easy as adopting a much healthier diet, taking a few supplements and not eating snacks. You might find the following path useful.
Obtain a Copy of "The Longevity Diet: Discover Calorie Restriction"
This book is a very good, easy introduction to the principles and simple ideas behind calorie restriction. Beyond that, it is a practical guide that will help you over a lot of the early pitfalls. It handily answers the "what exactly is it I eat?" question and offers some great tips for new practitioners.
Practice Eating a Better Diet First
While waiting for your book to arrive, you can start to shift your diet in preparation. Have a look at this resource for a class of diets known as "paleodiets":
The selling point of paleodiets is that they replicate the hunter/gatherer diet of our ancestors, and are therefore better for us. This is not an argument advocated here at all, but trying out a paleodiet is a great introduction to many of the strategies you'll adopt while on a calorie restriction diet. One thing that you will find out quite early on in your journey into calorie restriction is that you have to stop eating any significant amount of highly processed, rich, modern foods. These items are delicious, but heavy in calories and light in nutritional value.
In the US, you can walk into any corner store and eat 1500 Kcal of junk food (chips, chocolate, and so forth) at a cost of $10. You'll be hungry again a few hours later. That same $10 could feed you for two days if you buy vegetables, rice and tofu. You could eat 1500 Kcal each day and hardly be hungry at all. These two examples lie at the opposite ends of the spectrum, but most people eat many more "empty calories" (calories that do not provide vitamins and essential micronutrients) than they should. Adopting a paleodiet for a while is an easy way to start thinking seriously about what you eat, how you cook, and how you can better organize your eating habits. It's a smaller and more manageable step than leaping straight into calorie restriction.
If you were eating an unhealthy diet, you will probably notice the benefits of healthy eating within a few weeks. Your palate will become more sensitive to subtle tastes, you'll need less sleep, feel more alert, and mood swings will be diminished. Much of this stems from cutting the intake of processed sugars.